Field
This invention pertains to the cathodic protection of metallic surfaces. It is specifically directed to such systems including marine aluminum alloy anodes directly coupled to the structure. It provides a new assembly of components, which may be embodied in an anode structure, whereby the surface of the anode is maintained sacrificial. It further provides a surface mounting assembly which assures an excellent conduction path between the anode assembly and the protected metal surface.
State of the Art
Corrosion of metallic structures exposed to either a marine or soil environment has been a notable problem in the arts utilizing such structures. A great deal of research has been conducted in the public and private sectors involving the cathodic protection of various structures, for example, ship hulls and underground pipes. Various types of impressed current systems have been employed with considerable success, but they have the attendant disadvantage of high construction, installation and maintenance costs. Impressed current systems, while theoretically ideal for the protection of hulls, have not performed satisfactorily in service because of their delicate nature and somewhat sophisticated maintenance requirements. Direct-coupled sacrificial anodes offer the advantages of low cost for construction and installation as well as relative low maintenance. Such anodes as have heretofore been available for marine applications, are characteristically effective for a time, but tend soon to develop passive coatings which alter their surface potentials. The United States Bureau of Ships has developed a direct-coupled zinc anode of high purity zinc metal, known as Military Specification MIL-A-18001 which is presently regarded as the best available. Even this high purity anode, when directly coupled to a steel ship, tends to develop inert coatings on its surfaces after only a few weeks of exposure to sea water. The surface potential of the zinc is thereby lowered so that the anode becomes ineffective in protecting a ship's hull from corrosion. It is estimated that currently, sacrificial zinc anodes of one type or another are used in connection with in excess of 90 percent of the world's shipping. Sacrificial marine anodes of marine aluminum alloy are also commercially available.
Illustrative of the art generally dealing with sacrificial anodes, and in some cases zinc anodes in association with metal structures, are U.S. Pat. Nos. 3,870,615 (Wilson); 3,726,779 (Morgan); 3,485,741 (Booker); 3,425,925 (Fleischman); 1,984,899 (Smith); 3,048,535 (Sabins); 2,619,455 (Harris et al); 3,260,661 (Kemp et al); 3,047,478 (Marsh et al); 3,772,179 (Beer); 3,567,676 (Herrigel et al); and 2,779,729 (Jorgensen); and British Patents Nos. 11,216 (Morrison); 3205 (Casperson); and 852,154. A galvanic anode constructed of marine aluminum alloy is disclosed by U.S. Pat. No. 3,227,644 (Rutemiller).